The invention relates to an inorganic polymer material based on tantalum oxide, notably with a high refractive index and mechanically resistant to abrasion, its method of manufacture, in particular making use of precursors based on chlorinated derivatives of tantalum, and optical materials such as anti-glare materials and reflecting materials manufactured from this material.
The anti-glare materials and the reflecting materials are made up of an organic or inorganic substrate, covered with several layers, certain of which have the specific sought for properties. More precisely, interfering dielectric mirrors comprise a substrate, covered with a dielectric film which reflects one or more desired wavelengths, while at the same time having relatively low intrinsic absorption compared with metals traditionally used to produce mirrors.
The anti-glare or reflecting materials have a multitude of applications.
Hence the organic or inorganic substrates, that is to say particularly plastic or glass substrates, coated with an anti-glare film are of interest particularly in the following fields: ophthalmic products and video or architectural applications such as glass panels placed on the outside or on the inside of buildings. Apart from this, the anti-glare materials and the interfering dielectric mirrors can also be used in high energy lasers, solar applications, heating and photovoltaic applications or in integrated optical systems.
Methods of producing these anti-glare materials or interfering dielectric mirrors are already known from the prior art. These methods are mentioned below.
Furthermore, while in the ophthalmic sector, plastics such as polycarbonates, polyacrylates, polyallylcarbonates and others are particularly interesting, glass substrates are equally interesting, notably in the field of general optics and in the field of screens such as display screens.
It can be easily understood that because of about 4% reflection loss for each air-glass interface and the mean index for glass being 1.5, the losses for a complex optical system are often high.
Consequently, opticians have been seeking for a long time to create coatings with optical properties and notably anti-glare films, using physical deposition processes under vacuum, collectively known as PVD Technology (Physical Vapor Deposition).
Among these methods, there is simple or reactive spraying, simple or reactive evaporation by electronic or ionic heating with or without assistance, etc,.
Despite the excellent optical, chemical and mechanical qualities of the deposits, these techniques require sophisticated equipment which is heavy and costly and processes which are rather long. This is even more so when the surface area of components to be treated becomes large. The result is that such processes are generally poorly suited to cheap line production.
For example, only top of the range cathode tube screens for televisions are at present being given anti-glare coatings applied by PVD.
This is the reason why deposition processes using a gentle chemical route and in particular processes of deposition by the sol-gel route appear to be an interesting alternative to the physical processes of deposition under vacuum.
The method of deposition by the sol-gel route allows one to produce films on substrates which have various optical properties. Such a process when compared with the traditional processes of deposition under vacuum offers a number of advantages among which one may mention, deposition generally carried out at ambient temperature and at atmospheric pressure without having recourse to a thermal stage at temperatures which are too high, reduced capital costs and a simple and rapid method of application that allows for great flexibility of operation.
The deposition of metal or non-metal oxides with optical properties using a sol-gel method has been widely studied. It is apparent that the sol-gel systems or methods can be classified into two categories: polymer methods or systems and colloidal methods or systems.
Each system requires different preparations and operating conditions which depend on the properties of the desired treatment solutions and the nature of the oxide concerned.
The polymer system consists of using, as precursors, monomer, oligomer or low molecular weight species, in solution, with good molecular homogeneity and which are then converted into oxide, after application onto the substrate, by a firing step. The liquid deposited possibly changes in viscosity as evaporation of the solvent progresses until a gel is formed on the substrate. The solid network obtained still containing solvent is then converted into oxide by heating the system generally to temperatures up to 500xc2x0 C. One then obtains a dense, hard layer, that adheres strongly to the substrate. The conversion into oxide is generally accompanied by a large loss of mass made up of water and organic materials that brings with it a large reduction in the thickness of the layer. This induces high internal stresses, both tensile and compressive, in the deposit which can cause fine cracking of the coating in the case of thick mono- and multi-component films, that is to say films with a thickness that is greater than a few xcexcm.
For example, German patents DE-A-736 411 and DE-A-937 913 mention the use of hydrolysable compounds for the preparation of various interfering films. The major disadvantage of these methods resides in the indispensable thermal treatment between 500 and 600xc2x0 C. to convert the polymer intermediates into final dense ceramics. These high temperatures limit the choice for the nature of the substrate to be coated and complicate the industrial application.
The patent U.S. Pat. No. 2,466,119 describes a method of preparing reflecting and/or anti-glare multilayer films by hydrolysis and condensation of mixtures of halides of titanium and/or alkoxides of silicon. The control of the porosity of these layers is carried out by varying the temperature. However to obtain layers having good mechanical strength requires heating to temperatures which are very much greater than those that the usual plastics can tolerate, their thermal stability generally being 150xc2x0 C. as a maximum.
The patent U.S. Pat. No. 2,584,905 deals with the preparation of reflecting thin layers from alcoholic solutions of TiCl4 and silicon alkoxide. Here also, it is necessary to have resort to a heat treatment step that allows the oxides to be densified in a suitable way. In this method, the problems of fine cracks and spalling linked to the densification of the materials, considerably reduce the development of multi-layer build-ups with high reflection.
The patent U.S. Pat. No. 3,460,956 describes the preparation of reflecting films made of TiO2 from hydrosylates of tetra alkyl titanates, in an alcoholic medium. However, for effective conversion of the polymeric film into dense oxide, it must be heated to heating to temperatures of about 500xc2x0 C., which penalizes and can be damaging to any organic substrate.
The patents U.S. Pat. Nos. 2,768,909 and 2,710,267 describe the production of reflecting films made of TiO2 from alcoholic sols of an alkoxide of titanium, these sols being hydrolysable by atmospheric moisture. This approach also requires strong firing of the condensed intermediates and the layers obtained are not resistant to abrasion.
The patent U.S. Pat. No. 4,272,588 is concerned with the possibility of increasing the reflectivity of mirrors made of noble metals and the possibility of making them chemically passive, by the deposition of dielectric TiO2 and Ta2O5 layers arising from molecular precursors.
Such coatings are obtained by essential heating to about 400xc2x0 C.
Hence, the polymeric material generally used for thin optical layers with a high refractive index (between, for example, 1.9 and 2.1) is titanium oxide (TiO2). However in order to obtain layers that are mechanically resistant to abrasion, the densification must be carried out at high temperature close to 400xc2x0 C. which could not be considered for plastic substrates for example.
The document U.S. Pat. No. 4,328,260 describes a method and a composition to apply an anti-glare treatment and a grid onto solar cells which includes the application of a mask onto the surface of the cell, the application of a metal oxide paste (Ta, Ti, Nb, Y, Zr, Te) onto the mask, and the heating of the cell to a temperature of 300xc2x0 C. to 550xc2x0 C. to decompose the alkoxide and form the metal oxide.
The remaining surfaces are plated with nickel to form a metal grid. The application at the same time of an anti-glare coating and a grid causes problems which are posed in that document, which are fundamentally different to those of this application. Furthermore, the temperatures used to produce the metal oxide are very high and incompatible with a substrate such as an organic substrate. Apart from this, the application of a paste to a substrate does not allow precise control of the deposited thickness.
The document JP-A-55 010455 relates to the preparation of an anti-glare coating on a silicon substrate by deposition of a mixture of tantalum alkoxide and a complexing agent such as acetic acid and heating to a temperature of from 200 to 800xc2x0 C.
However, such a method has the disadvantages that the stability of the alkoxide solutions is very low, they have a high cost and the temperatures used are not suitable for all substrates.
The document by T. OHISHI et al. xe2x80x9cSynthesis and properties of tantalum oxide films prepared by the sol-gel method using photo-irradiationxe2x80x9d Journal of Non-crystalline Solids, 147, 148 (1992) 493-498 describes the preparation of thin dielectric layers of Ta2O5 by deposition of tantalum ethoxide solutions and exposure to ultra-violet rays. The disadvantages of such a method are linked to the use of tantalum alkoxide as precursor, and the thin layers prepared stem from unstable solutions which are subjected to photo-irradiation.
Finally, the document by T. J. REHG et al. xe2x80x9cSol-gel derived tantalum pentoxide films as ultra-violet anti-reflective coating for siliconxe2x80x9d, Applied Optics, 15.12.1989, Vol.28, N.24, p 5215-, describes a method of preparation of anti-glare coatings on silicon by deposition of a solution of tantalum penta-ethoxide and heat treatment at a temperature of from 300 to 1000xc2x0 C.
The other method or system of deposition by a sol-gel route is the colloidal method or system in which one uses dispersions of small particles, in particular oxides or fluorides, crystallized or amorphous, either by encouraging a germination-growth mechanism and then by stabilizing the system with a desired degree of nucleation, or by precipitation-peptization in a suitable solvent, to give colloidal suspensions, these suspensions forming what is called a xe2x80x9csolxe2x80x9d.
During the deposition, the evaporation of the solventxe2x80x94this being chosen to be sufficiently volatile to evaporate easilyxe2x80x94leads to an increase in the concentration of particles which, in most cases, precipitate onto the substrate.
The resulting coating is porous, with no internal stress and mechanically non-resistant to abrasion.
Examples of producing sol/gel layers by such a method are described notably in the patent application U.S. Pat. No. 7,148,458 (NTIS) corresponding to patents U.S. Pat. No. 4,929,278 and U.S. Pat. No. 4,966,812 and in patents U.S. Pat. No. 2,432,483 and U.S. Pat. No. 4,271,210.
The patent application U.S. Pat. No. 7,148,458 (NTIS) describes a method of depositing an anti-glare film on plastic substrates, consisting of synthesizing an ethanolic gel in the system SiO2xe2x80x94B2O3xe2x80x94Al2O3xe2x80x94BaO until a certain molecular complexity is obtained, and then of reliquefying this gel mechanically breaking certain interpolymer bridges. In this way, a porous film is obtained with a low refractive index (about 1.23), produced at ambient temperature, which allows it to be made suitable for plastic substrates; however this film only has a mediocre resistance to abrasion.
The American patents U.S. Pat. Nos. 2,432,483 and 4,271,210 disclose the possibility of using colloids of silica or alumina for the creation of dielectric anti-glare coatings, that permits the porosity of these coatings to be increased and therefore lower their refractive indices. If these methods have the advantage of being able to be implemented at low temperatures, the colloidal layers obtained have very low mechanical strength and are particularly sensitive to any physical contact.
In addition, the article entitled xe2x80x9cColloidal Sol-gel Optical Coatingsxe2x80x9d that appeared in xe2x80x9cThe American Ceramic Society Bulletinxe2x80x9d, Vol. 69, No. 7, pp. 1141-1143, 1990, describes a method of depositing thin layers by a sol-gel route using centrifugal surface application.
This article makes clear that using colloidal sol-gel suspensions and carefully choosing volatile solvents to constitute the liquid phase of the colloidal medium, it is possible to carry out treatments at ambient temperature, without excessive heating of the substrate. This technique therefore allows one to treat materials that are thermally fragile.
However, even the nature of these colloidal films, that is to say porous, implies low mechanical strength of these films, both from the point of view of abrasion, and adhesion to the substrate on which they are deposited. Such deposits do not tolerate any physical contact either touching or wiping, without being damaged. The only forces of cohesion which exist in these colloidal films are of the physical adsorption type and there are no chemical bonds whatsoever between the particles and the substrate or between the particles themselves.
The mechanical behavior can however be significantly improved by the addition of a binder between the particles. This binder, in truth a chemical interparticle xe2x80x9cjointxe2x80x9d can be of the organic, inorganic or hybrid type. It reinforces the mechanical cohesion of the system.
Thin optical layers based on colloidal silica (SiO2) are known from the prior art, from at least three documents that make reference to a significant improvement in their mechanical behavior.
The patent U.S. Pat. No. 2,432,484 discloses the use of a product composed of alcohol, a catalyst and tetraethyl orthosilicate and which uses a chemical binder between the colloidal particles in a way that reinforces the cohesion of the porous structure. This chemical binder is either applied onto the layer of colloidal silica already deposited, or incorporated into the treating medium (that is to say, the colloidal sol) and the whole is applied in a single treatment. Depending on the proportion of chemical binder used, the porosity of the colloidal deposit can remain virtually unchanged and in this way the optical properties can be preserved. The mechanical strength of the film reinforced in this way allows touching and wiping. Furthermore, a supplementary heat treatment of the coating at low temperature, that is to say about 100xc2x0 C., allows the strength to be increased even more. However, such a deposit remains vulnerable in the event of strong abrasive attack.
From an article by R.G. MUSKET et al. of the Lawrence Livermore National Laboratory of California that appeared in Appl. Phys. Lett., Vol. 52(5), 1988, a method is known of increasing the adhesion of the oxide/oxide interface using an ion beam. The authors describe a treatment by irradiation at 200 keV with helium ions He+, of anti-glare layers based on colloidal silica. This treatment allows one to improve the adhesion of the particles to one another and to the substrate, which ensures that the layer treated in this way has a normal resistance to optical cleaning (wiping) without any modification of optical properties. The explanation advanced for this phenomenon rests on a surface reactivity of the colloidal particles which is increased due to the ionic bombardment.
The French patent application No. 93 03987 of the 5th April 1993 from the CEA describes a method of improving the resistance to abrasion of thin layers with anti-glare optical properties by using alkaline reagents after deposition of the film. However, although such a method is carried out at ordinary temperature and pressure, the resistance to abrasion of such layers is insufficient for use by the public at large.
The French patent FR-A-2 680 583 from the CEA describes a material that has anti-glare properties, as well as hydrophobic properties and resistance to abrasion. This material comprises an organic or inorganic type substrate, covered successively by a layer of an adherence promoter produced in a material chosen from among the silanes, an anti-glare layer of silica colloid encased in a siloxane binder, an anti-abrasive layer of a fluorinated polymer. However, this material offers a spectral transmission window of a monolayer coating, of the order of only 100 nm. and a resistance to abrasion which is reasonable without being totally damage-proof.
The French patent application FR 2 682 486 from the CEA describes the preparation of dielectric mirrors with high resistance to a laser flux, by a method carried out at ambient temperature, which makes it suitable for organic substrates. The thin layers having the desired optical properties are prepared from colloidal suspensions, which are deposited by alternating a material with a low refractive index with a material with a high refractive index.
Nevertheless, the colloidal layers used are by nature porous which leads to a low refractive index compared with the index of a film of the same material in a dense form. Consequently, at equivalent reflectivity, it is necessary to stack a larger number of layers in order to make up for this difference in index, which implies a longer treatment, and means the optical coating becoming more fragile.
The French patent application FR 93 08762 from the CEA describes the preparation of composite materials with a high refractive index, characterized in that they include colloids of metal oxide encased in a polyvinyl polymer, soluble in an alcoholic solvent. The organic polymer encasing the colloid leads to a reduction in the residual open porosity between the oxide particles. The result is an increase in the refractive index of the deposited layer, an increase in the mechanical resistance to abrasion properties compared with the corresponding colloidal layer since the polymer serves as a binder between the particles and an improvement in the resistance to the laser flux.
However, the improvement in the mechanical resistance to abrasion properties of the layer obtained necessitates the use of layers of adherence promoters or layers of coupling agents. This increases the manufacturing time and the production costs. Furthermore, the mechanical resistance to abrasion properties remain inadequate particularly in relation to use by the public at large, for example, for the case of producing an anti-glare treatment for screens, notably cathode tube screens for televisions or other equipment.
Therefore the aim of the invention, amongst others, is to overcome the disadvantages of the prior art mentioned above and to provide a material, notably with a high refractive index, having good mechanical strength properties, that is to say among other things a good resistance to abrasion and satisfactory adhesion onto any substrate.
Another aim of the invention is to prepare materials with optical properties, using the material according to the invention, notably with a high refractive index.
The materials with optical properties are for example, materials that have anti-glare properties over a wide or narrow spectral band and among others, good hydrophobic and resistance to abrasion properties that imply because of this, easy cleaning, since the reflecting materials have properties of resistance to abrasion.
This aim and others have been achieved according to the invention by an inorganic polymer based on densified and cross-linked by a heat treatment at a moderate temperature or by exposure to ultra-violet rays. This material is notably a material with a high refractive index, and is, amongst other things, mechanically resistant to abrasion.
According to the characteristics of the invention, the material comprises more particularly, an inorganic polymer of tantalum oxyhydroxide, densified or cross-linked and generally including residual halide ions for example chlorides.
A polymer according to the invention is obtained from a molecular compound or precursor based on tantalum, soluble in a solvent, preferably an alcoholic solvent, which gives an inorganic polymer film (or layer) after densification or cross-linking, for example, by heat treatment and/or exposure to ultra-violet rays or other radiation.
According to the invention, the cross linking-densification heat treatment is generally carried out at a moderate temperature, not very high, namely for example less than or equal to 200xc2x0 C., preferably less than or equal to 150xc2x0 C., notably from 100 to 200xc2x0 C., and the molecular precursor is preferably an anhydrous compound, more preferably a halogen containing compound for example tantalum chloride.
Thanks to the characteristics of the invention, the molecular compound based on tantalum reacts with the water present in the ambient humidity and forms an inorganic polymer during the formation of the film. The densification step brings about cross-linking of this inorganic network, that is the formation of covalent tantalum-oxygen-tantalum chemical bonds. The result is an increase in the refractive index of the deposited layer, linked to the densification of the inorganic network, and an increase in the mechanical resistance to abrasion properties.
The material according to the invention is xe2x80x9cbased onxe2x80x9d tantalum oxide, that is to say that it can be constituted only of tantalum oxide or it can also include, apart form this oxide, at least one other metal or metalloid oxide, chosen preferably from among, silicon oxide, yttrium oxide, scandium oxide titanium oxide, hafnium oxide, thorium oxide, niobium oxide, zirconium oxide, lanthanum oxide, aluminum oxide and magnesium oxide in a proportion for example from 1 to 99%, preferably from 10 to 90%, by mass, with respect to the total mass of metal or metalloid oxides.
Such compounds allow the properties of the material based on tantalum oxide to be varied, in particular the refractive index and the resistance to abrasion.
It is therefore possible to obtain, for example, high indices with a polymeric material constituted only of tantalum oxide or with a polymeric material including apart from tantalum oxide, at least one other metal or metalloid oxide chosen from among the metal or metalloid oxides mentioned above, preferably with the exception of silicon oxide and magnesium oxide.
It will also be possible to obtain high medium or low indices with a polymeric material that includes, apart from tantalum oxide, for example at least silicon oxide and/or magnesium oxide.
The invention also relates to a method of preparation and of deposition of this polymeric material. According to the characteristics of the invention, this method comprises the steps consisting of:
preparing a solution (1) in a solvent (3) comprising a molecular compound based on tantalum called a tantalum molecular precursor
possibly mixing said solution (1) with a solution in a solvent of the same kind comprising a compound of a metal or metalloid other than tantalum or adding said metal or metalloid compound to said solution (1); in any case meaning that a solution (2) is obtained
depositing the solution obtained on a support to form a uniform layer of polymeric material, and
cross-linking, densifying this polymeric layer based on tantalum oxide by a heat treatment at a moderate temperature, possibly followed by an annealing step or a post-treatment heating step.
According to the invention, this cross-linking-densification is carried out, in particular, by a heat treatment at a moderate temperature, not very high, for example, from 120xc2x0 to 200xc2x0 C., preferably not exceeding 150xc2x0 C. and/or by exposure to UV rays notably of wavelength between about 180 and 280 nm, or by any other method of cross-linking desired at ambient temperature or at a moderate temperature.
Advantageously, the cross-linking temperature of the polymeric material, not exceeding 150xc2x0 C., and which can even be ambient temperature, in the case of exposure to UV or other radiation, the method is therefore applicable to substrates made of plastic material or any other material that does not tolerate high treatment temperatures.
Furthermore, in the case of cross-linking-densification by UV radiation, the method can be carried out for a significantly shorter period of time.
The invention also relates to an optical material, characterized in that it comprises a substrate of the organic or inorganic type covered by at least one layer of the polymeric material, previously described, based on tantalum oxide, with a high, medium or low refractive index and mechanically resistant to abrasion.
The invention also features such an optical material characterized in that it comprises in addition to a layer of polymeric material based on tantalum oxide with a high refractive index, at least one layer chosen from among
a layer with a low refractive index, for example, based on colloids of silicon oxide, calcium fluoride or magnesium fluoride encased or not in a siloxane binder, or silicon oxide in polymeric form
a layer with a medium refractive index, for example, a material based on tantalum oxide and another metal or metalloid oxide
an anti-abrasive layer, based for example on fluorinated silane.
According to the invention, the layers with low and medium refractive index are preferably densified-cross-linked polymeric layers, preferably under the same conditions as the layer with high refractive index.
Furthermore, the invention also relates to two particular types of optical materials, namely a narrow band or broad band anti-glare material, and a dielectric mirror.
The anti-glare material is characterized in that it comprises a substrate of organic or inorganic nature covered successively by
a layer of polymeric material based on tantalum oxide with a high refractive index and mechanically resistant to abrasion previously described,
a layer with a low refractive index based, for example on colloids of silicon oxide, calcium fluoride or magnesium fluoride encased or not in a siloxane binder or silicon oxide in polymeric form.
Such a material will be rather a xe2x80x9cnarrow bandxe2x80x9d anti-glare material but with an extremely high performance and suitable particularly for applications in the spectacle trade.
If the anti-glare material comprises, in addition, applied onto the substrate below the layer of polymeric material based on tantalum oxide with a high refractive index, a layer with a medium refractive index (lower layer), formed preferably according to the invention, by a polymeric material based on tantalum oxide and another metal or metalloid oxide, preferably silicon oxide, one then obtains a xe2x80x9cbroad bandxe2x80x9d anti-glare material; the difference in band width existing between on the one hand, a xe2x80x9cbroad bandxe2x80x9d anti-glare material and on the other hand, a xe2x80x9cnarrow bandxe2x80x9d anti-glare material is generally about 50% or more.
The anti-glare material can also comprise, on the layer with a low refractive index, an anti-abrasive layer, produced preferably according to the invention and based on a fluoro-organosilane (fluorinated silane).
It should be noted on the one hand that in the case of an application on an organic support, it is necessary to use either a material that is not very deformable, that is to say with a low coefficient of thermal expansion, or a plastic support, previously coated with a lacquer, preferably a suitable organo-silane lacquer that permits densification or cross-linking of the layer based on tantalum without inducing stresses.
The structure of this material xe2x80x9cwith three layersxe2x80x9d has been optimized in such a way that a maximum transmission optical response is obtained over a broad spectral range, that is to say, for example with a xcex94xcex of 300 nm and centered on 550 nm.
The refractive index formula to respond to this criterion is thereforexe2x80x94starting from the substrate : medium index/high index/low index. One then reduces the spectral reflection of the treated substrates, for example up to less than 1% between 400 and 750 nm and for example to less than 0.8% at 580 nm.
In addition, the presence of an anti-abrasive layer according to the invention, preferably based on a fluorinated silane, allow the anti-glare properties to be preserved while significantly increasing the resistance to abrasion.
In addition, the layer of fluorinated silane gives to the deposit an anti-adhesive and hydrophobic character that is particularly interesting since it facilitates the cleaning of the treated surface.
The anti-glare layers prepared in this way are homogeneous and free of crazing and/or internal cleavage planes. Consequently, the anti-glare film obtained is sufficiently elastic to tolerate weak torsion or deformation forces, when applied to a plastic substrate. Furthermore, this film resists a hot moist and saline atmosphere and leads to long life even after several successive immersions in boiling salt water (greater or equal to 10).
When it is applied to a glassy substrate, the broad band anti-glare coating according to the invention, for example with a band width of 300 nm, centered on 550 nm, prepared by a sol-gel method has remarkable mechanical strength properties and can therefore be applied in the context of use by the public at large for example by being applied to cathode tube screens for televisions.
Generally, the anti-glare material according to the invention, in effect and in a surprising manner, fulfils all of the demands required for such a use, namely:
a spectral reflection less than 0.8% at 580 nm.
a reflection less than 1% between 450 and 750 nm over the whole spectral width
a minimum angular dependence of the reflection
a mechanical strength defined by a resistance to severe abrasion in accordance to the US-MIL-C-0675-C standard, characterized by an absence of damage after 40 passes.
a chemical resistance characterized by a resistance to current maintenance products, to acids, bases and to organic solvents (ethanol, acetone, etc,.).
In addition, as will be seen below, the method of preparation of the anti-glare material according to the invention, that does not involve high temperatures, is simple and inexpensive.
The invention also relates to a reflecting material, characterized in that it comprises an organic or inorganic substrate, covered by at least a sequence of two layers comprising:
a layer with a low refractive index, analogous to that already mentioned above, formed, for example, by colloids of silicon oxide, calcium fluoride or magnesium fluoride encased or not with a siloxane or silicon oxide or magnesium oxide binder in polymeric form; and
a polymeric layer based on tantalum oxide with a high refractive index and mechanically resistant to abrasion previously described;
possibly an anti-abrasive layer.
The reflecting material obtained is a mono or polychroic passive dielectric mirror, that reflects wavelengths ranging from the near ultra-violet to the near infra-red.
The material based on tantalum oxide and with a high refractive index is particularly suitable for the production of interfering multi-layer mirrors. In effect, in order to obtain a given reflectivity, the number of layers necessary is a function of the ratio of refractive indices (in. the case of a reflecting stack of layers, a quarter of a wave with 2 refractive indices).
For the alternating deposition of a layer with a low refractive index (low index: nB) and a layer with a high refractive index (high index: nH), the number of layers required will be lower the greater the ratio (nH/nB). On the other hand, the spectral width is also a function of the difference between the two refractive indices.
Thanks to the use of the material based on tantalum oxide and with a high refractive index according to the invention, the number of layers necessary to obtain a given reflectivity is reduced, for example by a factor of 1.5 to 2 in relation to the use of a medium
refractive index. This allows a reduction in the manufacturing period and the risks of contamination while the method of manufacturing the multi-layer mirror can be carried out at ambient temperature or at a moderate temperature less than or equal to 150xc2x0 C. for example.
The reflecting material can also comprise a substrate covered with at least one layer with a low refractive index, already described above, and at least one layer with a medium refractive index analogous to that already described above for the anti-glare material, formed preferably, according to the invention, from a material based on tantalum oxide and another oxide of a metal or a metalloid, preferably silicon oxide or magnesium oxide.
The material obtained would then qualify as a xe2x80x9csemi-reflecting materialxe2x80x9d.
Similarly, by reversing the order of layers of the anti-glare materials described above, one also obtains reflecting or semi-reflecting materials.
The invention will be better understood on reading the following description of an embodiment of the invention, given by way of an illustrative example and being non-limitative.